Spectrum access optimization for self organizing networks

ABSTRACT

Improved techniques are provided for managing and optimizing network resources and spectrum access in a Self-Organizing Network (SON). A Spectrum Access System (SAS) collects network-related information from a plurality of network sources, such as base stations and user equipments (UEs), to perform optimization and organization across different networks, network operators, and network technologies. In some embodiments, the SAS may use the network information and a Radio Environment Map to optimize TDD synchronization in the SON. In other embodiments, the SAS may use the network information to populate a global Neighbor Relation Table. The SAS also may use the network information to optimize one or more network parameters, such as Physical Cell Identities or Root Sequence Indexes, antenna parameters, transmit power levels, handover thresholds, channel assignments, and so on, for use in the SON. Advantageously, the SAS&#39;s optimized network parameters may be used to improve network performance, reduce signal interference, and adjust to network failures in the SON.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/319,665, filed Apr. 7, 2016, and U.S. Provisional ApplicationSer. No. 62/331,810, filed May 4, 2016, each of which is herebyincorporated by reference in its entirety.

FIELD OF INVENTION

The present invention relates to techniques for shared spectrum accessin wireless networks and, more particularly, to methods, systems, andapparatuses for optimizing shared spectrum access in such networks.

BACKGROUND

Radio frequency (RF) spectrum is the foundation for many wirelesscommunications systems in use today, including radar and cellularcommunications systems. Specified frequency ranges, sometimes identifiedas bands or channels, in the RF spectrum may be allocated for use bydifferent entities, for different purposes, or in different geographiclocations. As used in this disclosure, “spectrum” refers to anyfrequencies, frequency bands, and frequency channels in the RF spectrumthat may be used or allocated for wireless communications.

Because the available RF spectrum is finite, frequency allocations inthe spectrum are highly valued and often highly regulated. In the UnitedStates, for example, the Federal Communications Commission (FCC) and theNational Telecommunication and Information Administration (NTIA)regulate and manage spectrum allocations, allotments, and assignments.Frequency allocation is the process by which the entire RF spectrum isdivided into frequency bands established for particular types ofservice. These frequency allocations are then further subdivided intochannels designated for a particular service or “allotment.” Assignmentrefers to the final subdivision of the spectrum in which a party getsone or more frequency assignments, in the form of a license, to operatea radio transmitter on specific frequencies within a particulargeographic location.

The system of spectrum allocation, allotment, and assignment is failingto keep pace with the increasing demand for spectrum. There is thereforea need to improve how the available spectrum can be efficientlyallocated, allotted, and assigned in the face of growing demand. Unlessotherwise noted, “allocation” is used in the present disclosure togenerally refer to the process by which spectrum is allocated, allotted,and assigned to licensed users.

In view of this increasing demand for spectrum, a dynamic spectrumaccess (DSA) system may be used to share available spectrum amongmultiple users. A DSA system, for example, may include a Spectrum AccessSystem (SAS) that manages access to a shared spectrum, such as the 3.5GHz band recently made available for commercial use in the UnitedStates. In another example, a DSA system may be used to share access tounlicensed spectrum, such as TV Whitespace. Coordinating and managingmulti-user access to a shared spectrum present challenges in a DSAsystem.

There has been research and development of computer-automated techniquesfor the optimization and organization of spectrum allocation instandalone wireless networks, e.g., Self-Organizing Network (SON)techniques in 3^(rd) Generation Partnership Project (3GPP) networks. ASON may comprise one or more interconnected standalone networks, e.g.,sharing access to at least one SAS. These standalone networks typicallyuse a single radio access technology, such as described in the 3GPPstandards for Long Term Evolution (LTE). Further, these networks areusually managed by a single operator, e.g., Verizon or AT&T, which hasan exclusive license to use a portion of spectrum in a geographicalarea.

As wireless demands grow, shared spectrum usage is becoming more common,e.g., in Television Whitespace (TVWS) bands and in the 3.5 GHz Federalband. In these environments, a SAS may control spectrum access amongusers assigned to different priority levels (or “tiers”) ofspectrum-access privileges. The SAS may implement spectrum managementpolicies for users in each tier. For example, the SAS may be configuredto protect spectrum usage by higher-priority “primary users” in sharedbands from harmful interference that would result from communications bylower-priority “secondary users.” As used herein, a “user” may refer toa user equipment (such as a mobile phone) or a person using a userequipment as will be apparent in context. In many cases where there arerelatively few primary users, spectrum usage by primary users is low, sosecondary users can dominate overall resource usage. Nonetheless, inregions with primary users, the SAS should ensure that any spectrumallocations to secondary users will not create unacceptable levels ofinterference with the primary users.

To assist the SAS with spectrum management, base stations servingsecondary users are often required by regulation (e.g., FCCrequirements) to share their operating parameters (e.g., location,antenna characteristics, desired operating power, air interfacetechnology, requested data rates) and measurements (e.g., neighboringbase station interference, overall interference, bit/block/frame errorrates, latencies, buffer status) with the SAS. Secondary users may alsoopt to share operating parameters and measurements with the SAS, sincesuch sharing may result in improved secondary user performance.

The SAS may employ a Radio Environment Map (REM) to manage spectrumaccess in one or more wireless networks. The REM describes thepropagation environment between each pair of devices (e.g., basestations or mobile users) in the overall shared spectrum system. A REMcan be as simple as a table of location, time, and energy detected. Itmay include items such as, but not limited to, location, time, day,strength of signal, whether users are mobile and when they are roaming,travel directions, and so on. This information can be combined withgeographic data such as terrain, tree cover, season of the year,elevation, potential tall buildings, and many other possible relevantitems that can affect radio signals. The REM may be used to trackhistorical and geo-located spectrum data. For example, the REM mayreflect that every day at 5 PM at a major office park, cell phone usagepeaks as people leave from work to head home. However, at 6 PM, theremay be little cell phone usage.

The SAS can create an accurate and high-resolution REM because it canleverage powerful and distributed processing (e.g., cloud-basedprocessing instances) and large databases (e.g., building data, floorplans, clutter models, terrain models, base station and mobile usermodels) for propagation and interference modeling, and may apply machinelearning based on measurements to refine these models. The SAS can useits REM to create a global and precise view of the shared spectrumenvironment.

It is desirable that the SAS manages multi-user spectrum access in amanner that minimizes signal interference between the users, basestations, and other devices communicating in the network. Undesiredinterference may result when multiple devices transmit at the same time,for example, in a Time Division Duplex (TDD) system. TDD operation isproposed for many shared spectrum applications given the need forsecondary-user uplink and downlink operation in a single frequencyrange. TDD operation generally involves the use of fixed-duration frameswhich include uplink and downlink time slots. For example, time divisionLTE (TD-LTE) uses 5 ms or 10 ms frames containing ten subframes (eachwith two time slots) that can be designated for uplink or downlink use.The uplink subframes are used for mobile users to communicate with thebase station, whereas the downlink subframes are used by the basestation to communicate with the mobile users. Other TDD systems mayemploy different frame structures, e.g., containing differentconfigurations of subframes and/or time slots. As used herein, a“subframe” may contain one or more time slots and is not limited to anyparticular implementation.

A significant concern with TDD operation is interference caused byunsynchronized base-station transmissions from other operators. A basestation's transmission power is typically much larger than thetransmission power of the mobile users. As a result, in an uplinktransmission from a mobile user to a target base station, interferencefrom another base station's transmissions can dwarf the mobile user'ssignal at the target base station's receiver. Such interference can becaused by base stations operating on the same frequency channels or onchannels that are not sufficiently separated in frequency (e.g., basestations operating in adjacent frequency channels 101 and 102 as shownin FIG. 1).

Single operator networks typically resolve this issue by havingsynchronized TDD networks where each base station transmits in the sametime slots. FIG. 2 illustrates frames 201A and 201B for communicatingwith base stations 202A and 202B, respectively, with synchronized uplink(“U”) and downlink (“D”) subframes 203, including special (“S”)subframes that provide a guard interval between transitions from D to Usubframes. The synchronized uplink subframes are reserved formobile-user transmissions. Therefore, during an uplink subframe, a basestation will only receive signals from mobile users, avoidinginterference from other base stations. This avoids the severeinterference that would result if the base stations were uncoordinated,e.g., if some base stations were transmitting while others werereceiving.

Synchronization is complicated because some networks desire a largerportion of the TDD frame reserved for downlink transmissions. Forexample, some networks may predominantly support mobile users desiringdownlink video streaming. In contrast, other networks may supportsymmetric uplink traffic and downlink traffic, e.g., voice networks. Airinterfaces such as WiMAX and LTE provide predetermined TDD frameconfigurations, with known distributions of uplink and downlinksubframes and/or time slots, and allow the network (e.g., via static ordynamic configurations) to select a known TDD configuration with a givenframe structure to accommodate expected traffic load for each network.

Further complicating TDD network synchronization is that some basestations may lack the ability to determine accurate timing. Frame timingis typically achieved using GPS, Precision Time Protocol (PTP), ornetwork-listen capabilities at the base station. However, some basestations may lack such timing capability and, hence, these base stationscannot align the beginning of their TDD frames with other base stations.

In scenarios with multiple networks, operators, and/or air interfaces,it is difficult to provide TDD network synchronization in sharedfrequency channels. Shared channel networks may lack interfaces betweenthem, making it difficult for them to share the network parameters andmeasurements necessary for TDD network synchronization. In addition,networks employing different radio interface technologies (e.g., WiMAX,LTE) may not have TDD frame sizes and configurations that can be alignedwithout significant uplink/downlink conflicts. Other shared-channel basestations may not have accurate timing capability.

SUMMARY

The present invention provides improved techniques for managing andoptimizing network resources and spectrum access in SONs containing oneor more wireless networks. In the disclosed embodiments, an SAS collectsnetwork-related information from a plurality of network sources, such asbase stations and user equipments in one or more networks and secondarynetworks, to perform optimization and organization across differentnetworks, network operators, and network technologies. The networkinformation may include, but is not limited to, measurements relating tosignal strength, quality, or interference, TDD configurationinformation, Random Access Channel (RACH) reports, network capabilities,network topology information, network traffic or loading information,radio link failure (RLF) information, network testing information (suchas Minimization of Driving Test (MDT) measurements), or other networkinformation provided to the SAS from base stations and mobile users.

In some disclosed embodiments, the SAS uses the network information tooptimize TDD synchronization for communications between user equipmentsand base stations in the SON, e.g., to reduce signal interference in thenetwork. In some embodiments, the SAS uses the network information tocreate (or update) a global Neighbor Relation Table (NRT) thatidentifies a base station's neighbor relations (e.g., information aboutother base stations) outside of the base station's local network. TheSAS may provide the global NRT to the base station to facilitatecross-network handovers between the base station and base stations inother networks.

Further, in some disclosed embodiments the SAS may use the networkinformation to optimize one or more network parameters, such as PhysicalCell Identities (PCI) or Root Sequence Indexes (RSI), assigned to basestations in the SON, or other parameters, such as RACH parameters,antenna parameters (such as antenna azimuth and tilt), transmit powerlevels, inter-frequency handover thresholds, channel assignments, and soon, assigned to base stations or user equipment. Advantageously, theSAS's optimized network parameters may be used to improve networkperformance, reduce signal interference, and adjust to network failuresin the SON. The SAS may employ cloud-based processing that furtherenables the SAS to rapidly and cost-effectively perform such centralizedorganization and optimization functions.

In other embodiments, the SAS may use a REM to enhance its ability tooptimize and organize spectrum and resource usage in the SON. The SASmay maintain a global REM to keep track of the network informationacross different networks, operators, and vendors. In some embodiments,the SAS inputs at least some of the network information in the REM to anobjective function, from which the SAS may determine assignments ofsecondary-user channels and transmit power levels. The objectivefunction may account for at least one of secondary-user interference,secondary-user bandwidth, secondary-user geographic coverage,secondary-user TDD frame configurations, cost of switching betweenfrequency channels, cost of secondary users with different radiotechnologies operating on the same channel (co-channel) or on adjacentchannels, cost of misaligned TDD configurations, or any other networkinformation stored in or derived from information in the global REM. Inthis context, each cost may be represented by one or more valuesindicative of a relative or absolute cost associated with a networkconfiguration.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this disclosure, illustrate various disclosed embodiments. Inthe drawings:

FIG. 1 is a schematic diagram illustrating exemplary signal interferencebetween uplink and downlink channels used for communicating in anetwork.

FIG. 2 is a schematic diagram of an exemplary TDD frame structure thatmay be used for communications between base stations and user equipmentin a network.

FIG. 3 is a schematic diagram of a three-tiered organization for the 3.5GHz federal band including a Spectrum Access System, Incumbent Systems,Priority Access License Systems, and General Authorized Access Systemsthat may be used in accordance with the disclosed embodiments.

FIG. 4 is a schematic block diagram of an exemplary networkconfiguration including a Spectrum Access System, Incumbent Systems,Priority Access License Systems, and General Authorized Access Systemsthat may be used in accordance with the disclosed embodiments.

FIG. 5 is a schematic diagram of exemplary TDD Configurations that maybe used for communicating between base stations and user equipment inaccordance with at least some of the disclosed embodiments.

FIG. 6 is a schematic diagram of an exemplary user equipment incommunication with multiple base stations that have been assigned thesame PCI.

FIG. 7 is a schematic diagram of an exemplary base station that receivesfrom a user equipment a PCI corresponding to multiple other basestations in the network.

FIG. 8 is a schematic diagram of exemplary Resource Blocks showingtime-frequency maps for multiple antennas at base stations assigned todifferent PCI values configured to communicate in an LTE network thatmay be used in accordance with at least some disclosed embodiments.

FIG. 9 is a schematic diagram of exemplary Resource Blocks showingtime-frequency maps for a single antenna at base stations assigned todifferent PCI values configured to communicate in an LTE network thatmay be used in accordance with at least some disclosed embodiments.

FIG. 10 is a schematic diagram of an exemplary set of base stationsconfigured with different Root Sequence Index (RSI) andzeroCorrelationZoneConfig values in an LTE network that may be used inaccordance with at least some disclosed embodiments.

FIG. 11 is a schematic block diagram of exemplary data that may beprocessed by an SAS to perform certain network optimizations inaccordance with at least some disclosed embodiments.

FIG. 12 is a schematic diagram of an exemplary latitude-longitude plotillustrating different geographic areas and signal strengthscorresponding to available frequency channels that an SAS may provide asgrant options to one or more base stations in accordance with at leastsome disclosed embodiments.

FIG. 13 is a schematic diagram of an exemplary set of information thatmay be exchanged between an SAS and base stations in accordance with atleast some disclosed embodiments of the invention.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

The following detailed description refers to the accompanying drawings.Wherever possible, the same reference numbers are used in the drawingsand the following description to refer to the same or similar parts.While several illustrative embodiments are described herein,modifications, adaptations and other implementations are possible. Forexample, substitutions, additions, or modifications may be made to thecomponents and steps illustrated in the drawings, and the illustrativemethods described herein may be modified by substituting, reordering,removing, or adding steps to the disclosed methods. Accordingly, thefollowing detailed description is not limited to the disclosedembodiments and examples. Instead, the proper scope of the invention isdefined by the appended claims.

LTE has been widely deployed in the U.S. In a shared-spectrum system,such as the three-tiered model adopted by FCC for the 3.5 GHz band asshown in FIG. 3, an SAS may be used to manage spectrum access byIncumbent Systems (also described as primary systems), such as militaryor federal government users, who previously have had accessed the 3.5GHz spectrum, and also Priority Access License (PAL) and GeneralAuthorized Access (GAA) users who are permitted to share access to thesame spectrum in accordance with FCC regulations. In many cases, suchshared-spectrum systems will likely implement LTE as their wirelesscommunication standard, and their LTE systems will coexist with non-LTEsystems. Descriptions of the disclosed embodiments are provided belowusing the example of an LTE system and the three-tiered model adopted bythe FCC, but those skilled in the art will readily appreciate that theinvention is more generally applicable and not limited to the exemplarysystems and standards disclosed herein.

FIG. 4 illustrates an exemplary shared-spectrum system 400 consistentwith the disclosed embodiments. System 400 includes a SAS 410 to whichall devices of the system connect, one or more LTE networks 406 andtheir associated LTE systems 407, non-LTE incumbent systems(“incumbents”) 408, one or more SASs 402, and/or other non-LTE,non-incumbent networks 412 and their associated systems 413. SAS 410communicates via network 404, which may comprise any suitable private orpublic network, such as the interne, a wide area network, a local areanetwork, an enterprise network, a virtual private network, or any othertype of network that permits exchange of information between variouscomponents of the system. Network 404 may be implemented as one or moreinterconnected networks through which one or more LTE secondary networks406 and non-LTE secondary networks 412 may communicate.

LTE secondary network 406 may comprise, for example, a cellular network,base stations (eNodeB), user equipment (UE), emergency vehicles, or anyother system 407 that does not have incumbent or primary status. In thethree-tiered model shown in FIG. 3, LTE secondary network 406 mayprovide network access to PAL and/or GAA users. In the example of the3.5 GHz band, non-LTE incumbents 408 may comprise systems used bybranches of armed forces, for example, U.S. Department of Defense airsurveillance radar systems, or devices used by other governmentoperators. Non-LTE, non-incumbent secondary network 412 may comprise,for example, Wi-Fi networks or other telecommunication networks usingWideband Code Division Multiple Access (WCDMA), Global System for MobileCommunication (GSM), or any other suitable technology.

SAS 410 coordinates and manages spectrum sharing among non-LTEincumbents 408, LTE systems 407 associated with secondary network 406,other SASs 410, and non-LTE, non-incumbent systems 413 associated withsecondary network 412, for example, by assigning spectrum to an LTEsystem 407 under secondary network 406 as requested, while at the sametime ensuring that the LTE system and non-LTE, non-incumbent systems 413under secondary network 412, and other communication systems managed byone or more SASs 402 do not interfere with the non-LTE incumbents 408.One or more sensors 414 may be used to monitor which frequency channelsare in use by users of the incumbent systems 408 and report any detectedspectrum usage to the SAS 410. Alternatively, at least one IncumbentSystem Database 416 may be used to determine the presence and parametersof some incumbent systems 408. One or more SASs 402 may facilitatespectrum sharing by notifying LTE systems associated with at least onesecondary network 406 and non-LTE, non-incumbent systems associated withat least one secondary network 412 regarding which frequencies they mayoperate on, when they should vacate certain frequencies, or at whichpower level(s) they may transmit.

SAS 410 and the various network elements, such as base stations and userequipment, and LTE systems 407 under secondary network 406, incumbentnon-LTE systems 408, and non-LTE, non-incumbent systems 413 undersecondary network 412, each may include necessary components tofacilitate wireless communication among them. Such components mayinclude antennas (or antenna arrays), transmitters, receivers, and/ortransceivers. They may also include one or more processors forprocessing and generating signals and memory for storing data andinstructions for execution by the processors.

LTE standards provide for uplink and downlink communications usingdifferent frequency bands in a scheme referred to as frequency divisionduplexing (FDD), and also provide for use of the same frequency band onboth the uplink and downlink but at alternating times, which is referredto as time division duplexing or time-division LTE (TD-LTE). For TD-LTE,the standards provide predetermined TDD configurations defining whichsubframes are used for uplink and downlink communications. FIG. 5illustrates several exemplary TDD configurations, e.g., with respectiveTDD Configuration identifiers, that may be assigned to an LTE frame.Some of the exemplary TDD configurations have more subframes allocatedfor uplink communications (“UL Dominant”) and others have more fordownlink (“DL Dominant”). As shown in FIG. 5, frequency channels 510 maybe configured to use different TDD configurations.

Given its central position in the shared spectrum environment andprocessing capabilities, the SAS 410 is well positioned to perform TDDsynchronization across different networks, operators, and airinterfaces. The SAS can create a Radio Environment Map based on, forexample, base-station and mobile-user measurements, base-station andmobile-user parameters, SAS-internal propagation and aggregateinterference modeling, information accessed from one or more databases,and information obtained from spectrum sensing. The REM, expecteduplink/downlink traffic load, and timing capabilities can be used togroup networks on a particular TDD configuration. Base stations that areadequately geographically separated can operate on the same frequencychannel (co-channel) or on closely spaced channels with different TDDconfigurations because base station-to-base station interference is low.The SAS can determine, e.g., using the REM, the geometric distanceneeded between networks (or groups of base stations) using different TDDconfigurations such that these networks do not significantly interferewith one another.

Further, the SAS 410 can segregate base stations with poor timingcapabilities onto frequencies that are not shared with base stationscapable of accurate timing estimation. In such exemplary embodiments,the base stations with poor timing will not degrade the performance ofbase stations whose TDD frames can be aligned and synchronized.

In addition, the SAS can leverage its knowledge of the radio technologyused by each network to best choose a TDD configuration for each networkthat reduces or minimizes base station-to-base station interference. Forexample, networks configured for WiMAX and TD-LTE networks can besynchronized by choosing similar TDD configurations in each of thesenetworks. TD-LTE Frame Configuration 1 (TDD Config. #1 in FIG. 5) hassimilar transmission periods to the WiMAX frame structure correspondingto a DL:UL ratio of 29:18. TD-LTE Frame Configuration 2 (TDD Config. #2)has downlink/uplink transmission periods that are similar to the 35:12WiMAX frame structure.

The TDD configuration groupings also may be chosen such that networkswith very different traffic loading can use frequency channels 510 thatare sufficiently separated in frequency to minimize interference. Forexample, with reference to FIGS. 4 and 5, SAS 410 may assign basestations that use UL-Dominant Channel 1 and DL-Dominant Channel 15 tonon-adjacent frequency bands, preferably sufficiently separated infrequency to avoid or reduce interference. Groups of base stations withonly small differences between their uplink/downlink traffic loading canuse TDD configurations with only a small number of mismatcheduplink/downlink subframes or time slots, e.g., less than (or equal to) apredetermined threshold. For example, with TD-LTE, groupings using TDDConfiguration #0 should operate far away in frequency from groupingsthat use TDD Configuration #5 (FIG. 5), as more than half of thesubframes in these TDD frame structures are mismatched. In contrast,groupings using TDD Configuration #1 can operate in frequency close togroupings that use TDD Configuration #6 since they only mismatch inSubframe Number 4.

The SAS 410 not only may compare different TDD configurations toidentify the number of mismatched subframes or time slots between them(e.g., one mismatched subframe between TDD Configurations #1 and #6),but also may compare frame structures of different types of networks(e.g., LTE and WiMAX) to identify the number of mismatched subframes ortime slots between their frame structures. The SAS may compare theidentified number of mismatched subframes or time slots with apredetermined threshold value, e.g., two, three, or four, to determinewhether the TDD configurations or frame structures are sufficientlysimilar to be used on the same frequency channel or on closely-spacedfrequency channels in network 404 or in any secondary network 406 or412. The SAS may make this determination, for example, based on whetherthe number of mismatched subframes or time slots is less than (or equalto) the predetermined threshold value.

The SAS's knowledge regarding the aggregate interference affectingprimary users may be used to enhance the TDD synchronization. Forexample, in areas where some channels are not available for secondaryuse, the TDD configurations of these channels can be considered in theselection of the TDD configurations and channel assignments for theavailable secondary-user channels to allow better performance byseparating any conflicting TDD configurations. In addition, the SAS'sknowledge about the interference caused by primary users upon thelower-tier base stations may be considered as well in selecting the TDDconfiguration.

In operation, the SAS may group networks (and base stations in thosenetworks) according to TDD configurations based on its Radio EnvironmentMap, expected uplink/downlink traffic demand, timing capability, and airinterface technology. Then, the SAS may separate the different groupingsin frequency in relation to the uplink/downlink frame mismatches intheir TDD configurations, e.g., more mismatches result in a greaterfrequency separation. In this manner, the SAS may use TDD configurationgroupings to minimize or reduce the overall interference in the SON.

In some disclosed embodiments, SAS 410 may provide network topologyinformation to base stations, user equipment, and/or other systems 406,408, and 412 to facilitate handovers between base stations in one ormore of network 404 and secondary networks 406 and 412. In LTE and otherwireless networks, each base station maintains a list of neighboringbase stations in a Neighbor Relation Table (NRT). The NRT also mayinclude a set of parameters for the base station, such as the basestation's Physical Cell Identity (PCI), Evolved Cell Global Identifier(ECGI), Tracking Area Code (TAC), and Public Land Mobile Network (PLMN)ID(s). The NRT is used by the base station to ensure smooth mobile-userhandovers between base stations and minimize dropped calls and dataloss.

The base station's NRT may be created from one or more base-stationmeasurements (e.g., network-listen operations performed by the basestation), information exchanged between base stations, and mobile-userreports sent to the base station. In single-network, staticenvironments, NRTs can be generated using a large number of base-stationmeasurements and mobile-user reports from locations covering most of thebase station's network coverage area and over a relatively long timeduration. However, in shared spectrum environments, it may not bepossible for the base station to determine neighbor relations with basestations in different networks and/or base stations with differentoperators. Further, users in shared-spectrum environments may need torapidly and frequently switch channels or adjust their transmit powerlevels to avoid causing harmful interference to primary users or to copewith increased congestion.

In accordance with some disclosed embodiments, the SAS can create andmaintain a global NRT starting with single-network reports of individualbase-station NRTs, which can also be referred to as local NRTs. The SAScan augment the single-network local NRT reports with additionalneighbor relations across networks and operators, e.g., based on the SASREM. Whenever a base station registers with the SAS or is directed bythe SAS to switch channels, the SAS may provide that base station with acopy of the global NRT. The global NRT may then be used by the basestation to immediately ensure smooth mobile user handovers between basestations and minimize dropped data and calls. The global NRT provided bythe SAS gives the base station much more rapid and accurate knowledge ofits neighbor relations, since the global NRT reduces the need forbase-station measurements, base station-to-base station exchanges, andmobile-user reports, to create and maintain the contents of its localNRT. The SAS-provided global NRT may also contain details of neighborrelations outside the base station's local network, e.g., secondarynetwork 406 in FIG. 4.

The SAS 410 may develop, maintain, and update a global NRT based onsingle-network reports of neighbor lists received from base stations innetwork 404 and in the secondary networks 406 and 412 and/or based onthe contents of SAS's REM. The SAS may forward the global NRT to a basestation upon the base station's registration/frequency reassignments inthe shared spectrum system.

In other disclosed embodiments, the SAS 410 may manage the PCIs assignedto base stations in network 404 and in secondary networks 406 and 412,which also may promote more efficient quality of service (QoS), handoverperformance, cell coverage, and cell-edge throughput in the network. Forexample, LTE has 504 possible PCI values that may be assigned to basestations (eNodeBs). Proper PCI planning is useful for managinginterference levels in LTE networks and reducing cellacquisition/synchronization times by user equipment. A proper PCIplanning is, above all, collision free: No two co-channel neighboringbase stations are allowed to have the same PCI. This is desired to avoidlong cell-acquisition and synchronization times (as colliding PCIscreate confusion among UEs) and high interference levels. FIG. 6illustrates an exemplary collision that may arise if UE 610 receives thesame PCI from adjacent base stations 620 and 630, i.e., neighboring basestations. In this scenario, the UE cannot rely on the PCI to distinguishbetween the base stations, e.g., in the event it needs to perform ahandover to one of these base stations.

A proper PCI planning is also confusion free: No two neighbors of a basestation are allowed to have the same PCI if they are co-channel.Confusion-free PCI planning is crucial for the success of handover asthere cannot be any ambiguity when a base station decides to handover aUE to one of its neighbors based on the PCI reported by the UE. Forexample, FIG. 7 illustrates confusion that may arise when UE 710 reportsto its current base station 720 (PCI 0) that it should handover to anadjacent base station 730 with PCI 5, but there is another adjacent basestation 740 also with PCI 5.

There are numerous other secondary constraints that one encounters whenassigning PCIs to neighboring co-channel base stations. Here, for thesake of brevity, only three exemplary constraints are described that anSAS 410 may consider in accordance with the disclosed embodiments of theinvention.

In some disclosed embodiments, when base stations are frame synchronizedin a secondary network 406, co-channel neighboring base stations shouldnot be assigned the same PCI values modulo 3, i.e., the neighboring basestations should not have PCI values that result in the same remainderwhen divided by three. This improves cell acquisition/synchronizationspeed. With the same PCIs modulo 3 (“PCI mod 3”), a UE receives the samePrimary Synchronization Sequences (PSSs) from multiple base stationsand, therefore, makes optimistic channel estimates. This, subsequently,reduces the UE's chance of successfully detecting the SecondarySynchronization Sequences (SSSs). Avoiding assignments of the same PCIvalues modulo 3 to neighboring base stations also reduces interferenceamongst Cell Specific Reference Signals (CSRSs) and Channel QualityIndicator (CQI) reports.

FIG. 8 illustrates exemplary Resource Blocks (RB) 810 that may be usedby a base station having Antenna Ports 0 and 1. In this example, each RBcontains Resource Elements (REs) 820 that may be occupied by CSRSs ordiscontinuous transmission periods (DTX). As can be seen, the locationsof the CSRSs and DTXs in a time-frequency map is a function of the PCIvalue because the frequency subcarriers are cycled every 3 PCI values.With the same PCI mod 3, these subcarriers interfere with each other.This reduces the signal-to-noise ratio in general, and negativelyaffects CQI values in the network 404 and secondary networks 406 and412.

Further, when base stations are frame synchronized and usingsingle-antenna transmissions in network 404 or in secondary networks 406or 412, co-channel neighboring base stations should not be assigned thesame PCI values modulo 6 (“PCI mod 6”) in accordance with otherdisclosed embodiments. Similar to the dual-antenna case in FIG. 8,avoiding the same PCI mod 6 for neighboring base stations reducesinterference amongst CSRSs and CQI reports. FIG. 9 illustrates exemplaryREs 920 occupied by CSRSs in a typical RB 910 for single-antennatransmission configurations. As can be seen, the location of CSRSs inthe time-frequency map is a function of the PCI value because thefrequency subcarriers used cycle every 6 PCIs. With the same PCI mod 6,these subcarriers interfere with each other and negatively affect thesignal-to-noise ratio, in general, and CQI values, in particular, in thenetwork 404 or in secondary networks 406 or 412.

In yet other disclosed embodiments, co-channel neighboring base stationsshould not be assigned the same PCI mod 30. Avoiding the same PCI mod 30for neighboring base stations may reduce inter-cell interference byensuring that neighbors use different Up-Link (UL) Reference Signal (RS)sequences.

To assist with PCI optimization, the SAS 410 may maintain amulti-vendor, multi-operator list of base station PCIs. The SAS can alsounfold the global NRT to obtain a larger picture of the “neighborships”(e.g., neighbors, next-nearest neighbors, next-next-nearest neighbors,etc.) amongst the base stations in network 404 and in secondary networks406 and 412. In this larger (“global”) picture of the overall networktopology, neighbors of neighbors are also known for each base station,although these important pieces of information would not ordinarily beavailable at the level of base stations. By combining the list ofbase-station PCIs and the unfolded global NRT, the SAS may solve theproblem of hidden nodes, prevent PCI collision/confusion, satisfyvarious other secondary constraints, and efficiently use cellgrouping/clustering techniques across vendors and operators in thenetwork 404 and in secondary networks 406 and 412. The SAS may suggestan optimized set of candidate PCIs, preferably with rankings, that itsends to the base stations, for example, upon the base stations'registrations and/or channel assignments.

Moreover, in cases where due to the density of deployments, PCIconfusion would become unavoidable, the SAS can create appropriate PCIblack-lists (i.e., PCI values to avoid) for base stations to improvetheir handover performance and cell coverage. In these disclosedembodiments, the base stations should not attempt to handover to anotherneighboring base station with a black-listed PCI.

Similarly, during substantial network changes, the SAS 410 may be bestpositioned to come up with a PCI re-selection scheme that minimizesservice disruptions. As discussed above, in some disclosed embodimentsthe SAS may leverage a multi-vendor, multiple-operator list of basestation PCIs to suggest one or more PCI values, and may further rankcandidate PCI values or identify black-listed PCI values, that itprovides to a base station, for example, when the base station registersor is assigned/reassigned a frequency channel via the SAS.

In accordance with other disclosed embodiments, the SAS 410 may provideoptimizations for a Random Access (RA) procedure by which a UE or othersystem attempts to initiate communications with a base station in any ofthe network 404 or in a secondary network 406 or 412. During the RAprocedure, the UE transmits a base station-specific “signature” to thebase station over a Random Access CHannel (RACH). In LTE, for example,there are 838 different possible RACH signatures available to all basestations. To reduce the UEs' cell-acquisition time, co-channelneighboring base stations should not have shared signatures. In the LTEstandard, each base station is assigned 64 signatures and UEs canrandomly select one of the base station's signatures. The UE knows howto generate the 64 signatures based on the base station's Root SequenceIndex (RSI) and zeroCorrelationZoneConfig value. The first signature isa Zadoff-Chu sequence generated based on base station's RSI. The UEgenerates up to 63 more signatures by cyclic-shifting the firstsignature by intervals determined by the zeroCorrelationZoneConfig.Depending on the zeroCorrelationZoneConfig, however, the shift intervalmay be large. Thus, the UE may need more than one RSI value to generateall 64 of the base station's signatures. In general, the larger the basestation's cell radius, the larger the shift interval, and, therefore,the larger the number of required RSI values. Proper RSI planning isdesired to minimize signature collisions by different UEs attempting tocommunicate with the same base station and, ideally, have collision-freesignature selections, which is not a trivial task.

FIG. 10 depicts three exemplary co-channel neighboring base stations,which in this example are Citizens Broadband radio Service Devices(CBSDs) for use in the 3.5 GHz spectrum. Base station 1010 (“CBSD a”)has a zeroCorrelationZoneConfig equal to 12 while base stations 1020 and1030 (“CBSD b” and “CBSD c”) each have zeroCorrelationZoneConfig valuesequal to 11. A zeroCorrelationZoneConfig value of 12 corresponds to ashift interval of 119. As such, there are [839/119]=7 signatures perRSI, and a UE needs [64/7]=10 RSI to generate all 64 RACH signatures forthe base station 1010. On the other hand, a zeroCorrelationZoneConfigvalue of 11 corresponds to a shift interval of 93, which translates into[839/93]=9 signatures per RSI. In this example, the UE needs [64/9]=8RSI to generate all 64 signatures for base stations 1020 and 1030.

With a pool of possible RSIs ranging from 0 to 25, one can assign RSI 0to base station 1010. This assignment, indirectly uses up RSIs 1 to 9 aswell to account for the 10 RSIs that are needed to generate all 64signatures of base station 1010. If one randomly selects another RSIfrom 11 to 25, say RSI 14 for base station 1020, then RSIs 15 to 21 arealso indirectly used up to account for the 8 RSIs that are needed togenerate all 64 signatures of base station 1020. But this selection isnot a good one because the remaining RSIs in the pool are 10 to 13 and22 to 25, and no matter which of these remaining RSI values is selectedfor base station 1030, there will always be overlap of signatures. Onecan easily see that a proper RSI planning assigns RSIs 0, 10, and 18 tobase stations 1010, 1020, and 1030, respectively.

As can be seen, the nature of RSI planning is very similar to PCIplanning. As such, to assist with RSI optimization, the SAS 410 maysimilarly maintain a multi-vendor, multi-operator list of base stationRSIs and zeroCorrelationZoneConfig values. By combining this list andits global NRT, the SAS may prevent RSI collisions, minimize partial RSIcollisions, and efficiently use cell grouping/clustering techniquesacross vendors and operators. In these disclosed embodiments, the SASmay suggest an optimized set of candidate RSI values, preferably with asuggested ranking of those RSI values, to provide to base stations, forexample, upon their registrations and channel assignments.

Similar to the case of PCI planning, during substantial network changes,the SAS 410 may be best positioned to come up with an RSI re-selectionscheme that minimizes service disruptions. Thus, the SAS may leverage amulti-vendor, multiple-operator list of base station RSIs to suggest anRSI, or list of ranked candidate RSI values, for example, when a basestation registers or is assigned a channel via the SAS in accordancewith some disclosed embodiments of the invention.

In yet other disclosed embodiments, the SAS 410 may provide other RACHoptimizations in any of the network 404 or secondary networks 406 or412. In these exemplary embodiments, the goals of RACH optimization areto achieve UL synchronization between a UE and base station, and obtainresources for network messages, such as a Radio Resource Control (RRC)Connection Request message. RACH optimization may be used in thefollowing situations:

Initial access from the RRC Idle state

RRC Connection Re-establishment

Handover

Down-Link (DL) data arrival when the UL is “non-synchronized”

UL data arrival when UL is “non-synchronized”

In accordance with some disclosed embodiments, the SAS 410 may leveragereports from UEs across LTE operators to reduce RA procedure latency andinterference by optimizing Physical Random Access Channel (PRACH)parameter settings. Single-operator networks do not have visibility andcontrol across networks to perform such an optimization. Specifically,in LTE, base stations may request capable UEs to send certain RACHReports to the base stations. Such a RACH Report may include the numberof RACH signatures that the UE transmitted during its most-recentsuccessfully completed RA procedure as well as a Boolean parameterindicating whether contention resolution was unsuccessful for at leastone of the UE's transmitted signatures during the most-recentsuccessfully completed RA procedure.

The SAS 410 may use these RACH Reports to optimize a PRACH configurationindex, PRACH signature groupings, a PRACH back-off timer, and PRACHtransmission power control parameters. The SAS's RACH optimizations mayaim for adjusting the number of RA access attempts by UEs, improving RAaccess delays, reducing interference among RA access attempts, and/orimproving UL interference. The SAS further may leverage one or morereceived UE RACH Reports across LTE operators to improve RACHperformance by adjusting various RACH parameters.

The SAS 410 also may perform mobility load balancing and robustnessoptimization in accordance with some disclosed embodiments. For example,using the REM and global knowledge of base-station and mobile-userparameters and measurements, the SAS may suggest handover parameters,e.g., trigger thresholds and hysteresis parameters, for optimum mobilityload balancing (MLB) and mobile robustness optimization (MRO). Optimizedhandover parameters can increase the overall capacity of the network 404and secondary networks 406 and 412 by helping to avoid scenarios where abase station is congested with heavy user traffic while one or more ofits neighboring base stations is relatively unloaded. The SAS also maysuggest an antenna tilt, transmit power level, and inter-frequencyhandover thresholds based on optimization routines that can take intoconsideration the base-station backhaul capability. To assist with thisoptimization, the SAS can leverage handover and load-balancingmeasurements and other measurements including radio resource usage,Hardware (HW) and Transport Network Load (TNL) load indicators, MDT,RLF, and backhaul capability.

In some embodiments, the SAS 410 may detect a network failure and makenetwork modifications necessary to compensate for the failure, forexample, leveraging its global knowledge of the shared spectrumenvironment. In such situations, the SAS may adjust base stationparameters, such as transmit power levels and antenna parametersincluding azimuth and downtilt. In these exemplary embodiments, the SASmay suggest to a base station experiencing a failure the availability ofother shared spectrum channels that the base station could switch to forimproved performance. The SAS also may help with detection andcorrection of hidden PCI confusion cases and keeping base station NRTsup-to-date by purging unused neighbor relations.

According to other disclosed embodiments, the SAS 410 may providecoverage and capacity optimizations in the network 404 and in secondarynetworks 406 and 412. Given its centralized location, the SAS canleverage its REM to cost-effectively and practically employ distributedprocessing and databases to perform coverage and capacity optimizations1110, as illustrated in the exemplary schematic diagram in FIG. 11.These optimizations can leverage measurements from base stations and themobile users that each base station supports. For example, the SAS'soptimizations can leverage network data 1120 provided by each basestation and/or determined via accessing databases, such as thoseproviding population and geographical density of mobile users in an areaof interest. The optimizations can also use geographic data 1130, suchas for propagation modeling (e.g., floorplans, buildings, terrain, landcategory, clutter), and other information obtained from one or moreincumbent databases 1140.

Using this data 1120, 1130, and 1140, and base station service requests1150 (e.g., also shown in FIG. 11), the SAS 410 may perform anoptimization to determine the base station channel and transmit powerassignment. The SAS may take into consideration the REM it calculatesbased on measurements, base station parameters, and propagation modelingto perform this optimization. The SAS also may use measurements andmachine learning to refine its REM to better estimate basestation-to-base station interference, base station-to-mobile station,and mobile station-to-mobile station interference.

While the 3.5 GHz shared spectrum operation in the exemplary embodimentsmay support SAS-automated coverage and capacity optimization, thoseskilled in the art will recognize the described methods could be appliedin other systems having shared frequency bands. The 3.5-GHz SAS isrequired to protect Priority Access License (PAL) base stations andmobile users from interference from lower-tier users, such as GAA basestations and their mobile users. In some embodiments, the aggregateinterference from GAA users should be controlled to be less than orequal to −80 dBm/10 MHz in a PAL Protection Area (PPA) and onfrequencies where the PAL users operate. The PPA surrounds PAL basestations and their mobile users.

The SAS 410 can use an objective function to optimally assign GAA usersto frequency bands so they satisfy the PAL protection requirements andprimary-user constraints, and further achieve any desired GAAcoexistence optimization. For example, the objective function may beselected to minimize overall GAA-to-GAA interference (e.g., asdetermined using the REM) and maximize overall GAA bandwidth andcoverage while satisfying PPA and primary-user constraints.

The objective function can also weigh the cost of switching frequenciesin the GAA assignments and attempt to place base stations with differentradio technologies (e.g., WiMAX, LTE) far apart in frequency so as tonot degrade their measurement capabilities or cause interference due tomisaligned TDD configurations and framing. All these considerations canbe captured in the objective function used for GAA channel and powerlevel assignments.

In those disclosed embodiments that use such an objective function, theSAS may indicate to each GAA user the aggregate interference that userwould be permitted to generate over a certain geographical area and eachof a range of channels. The GAA user may then select, from theSAS-provided candidates, an actual channel that it would like to operateon. For example, as illustrated in FIG. 12, the SAS may suggest that theGAA user consider as options operating on Channel 2, 4, 5, or 11. TheGAA user would have to ensure that the aggregate interference from itsusers would be within −91 dBm on Channel 2 in an area defined by alatitude/longitude region or polygon (e.g., shown in the exemplary plotof “lat” and “long” in FIG. 12).

In one exemplary implementation, the SAS explicitly or implicitlypartitions its interaction with base stations and the associated SONsystem in the following phases:

Initial Spectrum Grant and Configuration for a base station: Onreceiving a Grant Request from a base station, the SAS uses informationprovided from the base station or SON or Element Management System(EMS)/Network Management System (NMS) that includes, but is not limitedto:

-   -   1. that particular base station's (eNodeB's) technical        capabilities such as receiver technology, supported operation        modes, timing capabilities;    -   2. REM measurements from the base stations;    -   3. REM information already available or provided by SON        pertaining to surrounding base stations;    -   4. network loading conditions and patterns;    -   5. traffic related information; or    -   6. services that the base station needs to support.        The SAS uses this information to calculate and communicate to        the base station the frequency, power, and operations        configurations described in earlier sections.

Steady State operation: Based on periodic or trigger-based push or pullof different indicators of performance, such as measurements fromsystems in the SON or base stations or EMS/NMS, the SAS providesoptimization guidance to the SON in terms of frequency, power, andoperations configurations, as described in earlier sections. Suchoptimizations may be performed on a relatively longer-term time-scaleand may be targeted to adapt to changing aspects of the networks, suchas loading on the network, etc.

Conflict resolution: Every once in a while, based on performance metricssuch as persistently high reported interference levels, throughputissues, RACH failures etc., the SAS may conclude that there is aconflict situation which is significantly impacting the performance ofone or more base stations, and the SAS switches to a more urgent actionin terms of configuration, frequency, and/or power recommendations. Insuch conflict situations, the SAS may iterate to get the affected basestation or base stations working well again, but in those embodimentssuch iterations may be made more frequently compared to steady stateoptimizations.

In the event that such conflict resolution methods are not found toalleviate the performance degradation of one or more base stations, theSAS may decide to re-assign one or more CBSDs to a part of the spectrumthat has been put aside for such scenarios, where the base stations thatare treated such may be deemed deficient in receiver technology or othernecessary technical capabilities such as timing, or may have beencompromised. In such disclosed embodiments, this part of the spectrum isless managed in terms of performance primarily as a function ofinsufficient base station capabilities as deemed by the SAS.

Further to these illustrative disclosed embodiments, the SAS may alsodecide to use a guard band between the part of the spectrum where basestation(s) with insufficient capabilities operate and the remainingspectrum where the more capable or healthy base stations operate andwhere there are better performance guarantees. The SAS may also supportmethods by which a base station that ends up in the part of the spectrumwith lesser performance guarantees, provides, after some correctiveactions, measurements or information that enables the SAS to decide thatsuch a base station is ready to be re-allocated back to the part of thespectrum that has better performance guarantees.

Accordingly, in some disclosed embodiments of the invention, the SAS 410may leverage its cross-network, cross-vendor, and cross-operator globalREM and an objective function to assign secondary-user channel andtransmit power levels. The objective function may include secondary-userinterference, secondary-user bandwidth, secondary-user coverage, cost ofswitching channels, cost of secondary users with different radiotechnologies operating co-channel or on adjacent channels, or cost ofmisaligned TDD configurations and framing.

FIG. 13 illustrates an exemplary flow of information that may beexchanged between the SAS 410 and base stations in accordance with atleast some of the disclosed embodiments herein. The SAS may sendinformation including, but not limited to, one or more of channel-grantoptions, suggested TDD configurations, PCI, RSI, global NRT, handoverparameters, transmit power levels, and conflict resolution informationto at least one base station in network 404 or in any secondary network406 or 412 in accordance with the disclosed embodiments. In someexemplary embodiments, the base stations may select a particular TDDconfiguration and/or choose a grant option (segment) to report to theSAS. The base stations may send information including, but not limitedto, one or more of registration requests, measurements (e.g., ReceivedSignal Strength Indicators (RSSI), Reference Signal Received Power(RSRP), Reference Signal Received Quality (RSRQ), bit or block errorrates (BLER), load balancing measurements, handover parameters, RACHReports, RLF information, MDT information, and other measurements), TDDconfiguration, PCI, and local NRT information, network capabilities,grant requests, segment selections, conflict information, or any otherinformation in accordance with the disclosed embodiments describedherein.

The exemplary SAS 410 described in the disclosed embodiments may beimplemented in hardware, software, or any combination of hardware andsoftware. A person of ordinary skill in the art will appreciate that theSAS may comprise any standalone or embedded general-purpose orspecial-purpose computer system that may be configured to operateconsistent with the disclosed embodiments, and also may comprise one ormore cloud-based services distributed over the network 404 and/orsecondary networks 406 and 412. For example, the SAS may provide certainfunctions or services that are at least partially performed on a cloudplatform (such as one or more remote servers) in communication with theSAS. By performing one or more of the optimizations for shared spectrumsystems described above, the SAS 410 can provide SON services andfunctions for the network 404 and secondary networks 406 and 412 in theexemplary disclosed embodiments herein.

While illustrative embodiments have been described herein, the scope ofany and all embodiments having equivalent elements, modifications,omissions, combinations (e.g., of aspects across various embodiments),adaptations and/or alterations as would be appreciated by those skilledin the art based on the present disclosure. The limitations in theclaims are to be interpreted broadly based on the language employed inthe claims and not limited to examples described in the presentspecification or during the prosecution of the application. The examplesare to be construed as non-exclusive. Furthermore, the steps of thedisclosed routines may be modified in any manner, including byreordering steps and/or inserting or deleting steps. It is intended,therefore, that the specification and examples be considered asillustrative only, with a true scope and spirit being indicated by thefollowing claims and their full scope of equivalents.

1-27. (canceled)
 28. A method for optimizing Physical Cell Identity(PCI) across multiple cellular operators in a shared spectrum systemcomprising one or more incumbent and non-incumbent systems, the methodcomprising: using, by a Spectrum Access System (SAS), Physical CellIdentity (PCI) information contained in a global neighbor relationstable (NRT) to maintain a list of PCIs assigned to a plurality of basestations, wherein the SAS is separate from each of the incumbent andnon-incumbent systems, and wherein the SAS has no control over the oneor more incumbent systems; determining, by the SAS, an optimized set ofcandidate PCIs based on the global NRT and the list of PCIs assigned tothe plurality of base stations; ranking, by the SAS, the optimized setof candidate PCIs in a list; and sending, by the SAS, the ranked list ofPCIs to one or more base stations of the plurality of base stations uponregistration, frequency assignment, or frequency reassignment of the oneor more base stations.
 29. The method of claim 28, further comprisingusing, by the SAS, the global NRT to determine whether PCIs assigned toadjacent base stations of the plurality of base stations will negativelyaffect network performance.
 30. The method of claim 28, wherein networkparameters assigned to the plurality of base stations comprise a RootSequence Index (RSI) and a zeroCorrelationZoneConfig value, and whereinthe method further comprises using, by the SAS, the global NRT and thezeroCorrelationZoneConfig value to determine whether RSI assigned toadjacent base stations of the plurality of base stations will preventRSI collisions.
 31. The method of claim 28, further comprising ranking,by the SAS, a list of proposed RSI values of the plurality of basestations to send to the one or more base stations.
 32. The method ofclaim 28, further comprising preventing, by the SAS, an assignment of asame PCI to co-channel neighboring base stations.
 33. The method ofclaim 28, further comprising generating, by the SAS, a PCI blacklist forthe plurality of base stations, the PCI blacklist comprising a list ofPCI values to avoid to improve handover performance and cell coverage.34. The method of claim 28, further comprising sending, by the SAS, acopy of the global NRT to the one or more base stations uponregistration or frequency reassignment of the one or more base stations,wherein the global NRT identifies information about base stationsoutside of a network associated with the one or more base stations. 35.The method of claim 28, wherein the global NRT comprises neighborrelation details of a plurality of base stations in a plurality ofnetworks.
 36. The method of claim 28, further comprising: collecting, bythe SAS, network information from one or more base stations and one ormore user equipment (UE); and updating the global NRT based on thecollected network information.
 37. The method of claim 36, wherein thecollected network information comprises at least one of signal strengthmeasurement, signal quality, interference, Time Division Duplex (TDD)configuration, Random Access Channel (RACH) reports, networkcapabilities, network topology information, network traffic or loadinginformation, radio link failure (RLF) information, or network testinginformation.
 38. A Spectrum Access System (SAS) for optimizing PhysicalCell Identity (PCI) across multiple cellular operators in a sharedspectrum system comprising one or more incumbent and non-incumbentsystems, the SAS comprising: one or more processors; a memory storinginstructions that when executed by the one or more processors cause theSAS to: use Physical Cell Identity (PCI) information contained in aglobal neighbor relations table (NRT) to maintain a list of PCIsassigned to a plurality of base stations, wherein the SAS is separatefrom each of the incumbent and non-incumbent systems, and wherein theSAS has no control over the one or more incumbent systems; determine anoptimized set of candidate PCIs based on the global NRT and the list ofPCIs assigned to the plurality of base stations; rank the optimized setof candidate PCIs in a list; and send the ranked list of PCIs to one ormore base stations of the plurality of base stations upon registration,frequency assignment, or frequency reassignment of the one or more basestations.
 39. The SAS of claim 38, wherein the instructions whenexecuted by the one or more processors further cause the SAS to use theglobal NRT to determine whether PCIs assigned to adjacent base stationsof the plurality of base stations will negatively affect networkperformance.
 40. The SAS of claim 38, wherein network parametersassigned to the plurality of base stations comprise a Root SequenceIndex (RSI) and a zeroCorrelationZoneConfig value, and wherein theinstructions when executed by the one or more processors further causethe SAS to use the global NRT and the zeroCorrelationZoneConfig value todetermine whether RSI assigned to adjacent base stations of theplurality of base stations will negatively affect network performance.41. The SAS of claim 38, wherein the instructions when executed by theone or more processors further cause the SAS to rank a list of proposedRSI values of the plurality of base stations to send to the one or morebase stations.
 42. The SAS of claim 38, wherein the instructions whenexecuted by the one or more processors further cause the SAS to preventan assignment of a same PCI to co-channel neighboring base stations. 43.The SAS of claim 38, wherein the instructions when executed by the oneor more processors further cause the SAS to generate a PCI blacklist forthe plurality of base stations, the PCI blacklist comprising a list ofPCI values to avoid to improve handover performance and cell coverage.44. The SAS of claim 38, wherein the instructions when executed by theone or more processors further cause the SAS to send a copy of theglobal NRT to the one or more base stations upon registration orfrequency reassignment of the one or more base stations, wherein theglobal NRT identifies information about base stations outside of anetwork associated with the one or more base stations.
 45. The SAS ofclaim 38, wherein the global NRT comprises neighbor relation details ofa plurality of base stations in a plurality of networks.
 46. The SAS ofclaim 38, wherein the instructions when executed by the one or moreprocessors further cause the SAS to: collect network information fromone or more base stations and one or more user equipment (UE); andupdate the global NRT based on the collected network information. 47.The SAS of claim 46, wherein the collected network information comprisesat least one of signal strength measurement, signal quality,interference, Time Division Duplex (TDD) configuration, Random AccessChannel (RACH) reports, network capabilities, network topologyinformation, network traffic or loading information, radio link failure(RLF) information, or network testing information.